As a promising material of renewable oil industry, biodiesel is a long chain fatty acid ester produced from renewable oil through transesterification reaction. Biodiesel is a novel, pollutionless and recoverable energy source. The combustion performance of biodiesel is comparable with the conventional petroleum-based diesel, while after combustion, the content of harmful substances in the exhaust gas decreases approximately by 50% as compared with the conventional petroleum-based diesel. By far, wide attentions have been drawn to the studies and applications regarding biodiesel.
Currently, biodiesel is manufactured mainly through chemical methods. Particularly, vegetable oils and/or animal fats are used as a source of long chain fatty acids and a transesterification reaction between the long chain fatty acids and some short chain alcohols, such as methanol or ethanol, is carried out in the presence of an acid or base catalyst, and short chain ester of the fatty acids are obtained. However, some inevitable disadvantages exist in chemical methods as follows:
(1). Free fatty acid and water contained in the renewable oil raw material severely spoil the reaction;
(2). Emulsion is undesirably formed due to the poor solubility of alcohol in renewable oils, and the subsequent treatment steps are complicated;
(3). As required by the process, the amount of short chain alcohol used is much more than the reaction molar ratio, and the evaporation/reflux of the excess short chain alcohol leads to increased energy consumption.
In contrast, synthesizing biodiesel through a biological enzyme method has the following advantages: mild reaction conditions, non-toxic emissions, and enzyme-catalyzed reactions are not affected by the free fatty acid and small amount of water contained in the renewable oil raw materials. Therefore, bio-methods are consistent with the requirement of developing Green Chemistry, and thus have attracted more and more attention.
However, when compared with the chemical process, there exist some problems in the biological enzyme process. In an article entitled Enzymatic alcoholysis for biodiesel fuel production and application of the reaction to oil processing (Journal of Molecular Catalysis B: Enzymatic, 2002, 17:133-142) written by Shimada Yuji et al, it is reported that firstly, the poor solubility of short chain alcohols, such as methanol and ethanol, in the renewable oil raw material is unfavorable for the reaction; and secondly, the presence of the excess alcohol may lead to severe deactivation of the enzyme. Therefore, during the process of biodiesel production by the biological enzyme method, batch addition of short chain alcohol is generally adopted to moderate the poisonous effect imposed on the enzyme. However, this process is complicated in operation and needs long reaction time. Additionally, a short chain alcohol is used as an acyl acceptor, and byproduct glycerin is produced during the course of reaction, then the hydrophilic glycerin readily adheres to the inner pores and the outer surface of the immobilized enzyme forming a “shield” on the active sites of the enzyme, and severely affecting the reactivity of the enzyme. Furthermore, the yield of biodiesel synthesized by the enzyme process is relatively low compared with that by the conventional chemical process. To solve above-mentioned problems in a biological enzyme process, some scholars have tried to produce biodiesel in organic solvent reaction system. It is reported in an article entitled Enzymatic alcoholysis for biodiesel fuel production and application of the reaction to oil processing (Journal of Molecular Catalysis B: Enzymatic, 2002, 17:133-142) written by Shimada Yuji et al that, some relatively strong hydrophobic organic solvents, such as hexane, cyclohexane and petroleum ether can be used as reaction medium. These hydrophobic solvents can dissolve renewable oil very well, thus promoting the reaction to some extent. However, because these highly hydrophobic solvents can not dissolve lower carbon alcohol, such as methanol, as well as the byproduct glycerin effectively, the reactivity and lifetime of the enzyme, and the yield of biodiesel can not be improved notably.